As silicon is reaching its performance limit, other semiconductor materials are drawing attention as potential device materials for integrated circuits. 4H-SiC (silicon-carbide) materials have a high bandgap and therefore represent one promising alternative to silicon. However, 4H-SiC presents several design challenges. For example, it is difficult to form high performance ohmic contacts on 4H-SiC substrates. No metal has the appropriate work function to form a low-resistance contact with silicon-carbide. Transition metals, such as nickel (Ni), cobalt (Co), iron (Fe), etc., are used to address this issue. These transition metals are deposited on the silicon-carbide surface and annealed in a high temperature (e.g., ≧800° C.) process. The high temperature annealing forms a metal-silicide along with a layer of carbon on the substrate. The carbon layer has a decreased work function and thus provides a tunneling Ohmic contact.
One challenge associated with using transition metals relates to the structuring of these metals to form the contact structure in the intended location. Lift-off techniques can be used to structure the transmission metals. According to a lift-off technique, a dissolvable mask (e.g., a photoresist mask) is formed on the substrate and structured so that the openings in the mask correspond to the desired contact location. A layer of the transition metal is deposited on the substrate so as to conform to the mask. The mask is then dissolved, thereby removing the portions of the metal layer that adhere to the mask.
The above described lift-off technique has notable drawbacks. For example, it is difficult to precisely control the removal of the transition metal layer along a defined boundary. In many cases, a small amount of metal remains present after the lift-off process is performed. This metal can contaminate devices regions, such as the gate oxide. This problem is particularly pronounced in smaller cell pitch devices. As a result, these devices suffer from higher defect density. In sum, lift-off techniques negatively impact manufacturing yield for SiC based devices.